1. Field of the Invention
This invention relates to particle or light beam systems used in the manufacture of microminiature electronic devices (integrated circuits) and is particularly directed to a new and improved writing technique for raster scan beam lithography system; the primary object of this invention is to provide a multiple pass writing strategy for a raster scan lithographic system without reduction in throughput, and thereby provide an optimal platform for implementation of known techniques for improving both lithography quality and throughput.
2. Prior Art
Lithographic systems using a controllable electron beam, sometimes called E-beam machines, for the fabrication of integrated circuits are old in the art; one such system is described at length in the U.S. Pat. No. 3,900,737 to Collier et al. and another in the U.S. Pat. No. 3,801,792 to Lin.
In these machines, a medium of resist (or photosensitive) material upon which the electron beam is to perform its writing operation is positioned on a motor driven stage which is moved continuously and in synchronism as the beam is scanned in a raster fashion (a raster scan) in a direction perpendicular to the stage motion. In practice, the diameter of the round electron beam spot, also called a "gaussian spot" or "pixel", focused on the resist layer, is of the order of (but not necessarily equal to) the writing address dimension (or address unit, au) of the cartesian grid on which it is written. Adjacent rows of pixels in the stage travel direction define the width of a "feature" and the length of the feature is formed by a number of pixels in the raster scan direction. In practice, adjacent "on" pixels in the same raster scan are not written separately but "scanned" with the beam on until an "off" pixel is encountered. For the purposes of this disclosure, descriptions are given in terms of the normal full pixelization representation. The pattern on the resist defined by the beam scan and by the stage movement is determined by the system control equipment and software therefor.
Prior art teaches that repetitive imaging strategies for raster scanning lithographic systems are desirable. Repeated writing of scanlines placing pixels in nominally the same location has long been used to increase the dose of energy delivered to the patterned area on the substrate. The higher dose allows the use of less sensitive but higher resolution resist coatings for improved lithography. Additionally, the redundancy averages out some of the random errors in the lithographic system. Further improvement may be obtained by offsetting the data within the raster scanline so that the repeated image spots (pixels) are still written redundantly at nominally the same substrate location, but from a different portion of the scanline. Lithography is improved because of averaging of systematic errors of the lithographic system. This is described in "Image Quality Enhancements for Raster Scan Lithography", L. Rieger, et al, 1988 SPIE Santa Clara Symposium. With a conventional writing technique, throughput is disadvantageously reduced proportional to the number of passes required for the repetitive imaging.
Other pertinent prior art is disclosed in "RASTERIZING SYSTEM UTILIZING AN OVERLAY OF BIT-MAPPED LOW ADDRESS RESOLUTION DATABASES" U.S. Pat. No. 4,879,605. to Warkentin, et al. A multi-pass writing technique is disclosed which requires multiple edge definitions achieved by varying the dosage at an edge dependent upon the desired location of a given edge. With a normal raster scan writing technique, feature edges are logically defined in the obvious manner by a row of exposed or "on" pixels at the feature edge. Incorporation of nonhomogeneous edge definitions into the writing technique requires more stringent control over other lithographic system and image development parameters to make all edges appear alike and resolve to the proper location when the image is developed.
It is further known that a large spot (pixel) overlapped by 40-50% on a smaller writing grid may be used to produce excellent lithography results with a raster scanning lithographic system. Overlapping of large spots in this manner has been used in single pass writing technique to increase dosage over that which is attainable without overlapped spots and without the attendant reduction of throughput required by the repeated scan method described above.